The present invention relates to a method and an apparatus for generating an ion current which occurs as a direct current between a central electrode and one or more side electrodes of a spark plug of a spark ignition engine which is supplied repeatedly with an ignition voltage. Such a method is generally described in U.S. Pub. No. 2011/0016952 A1.
With the known method, an ion current is generated that flows as a direct current between a central electrode and one or more side electrodes of a spark plug of a spark ignition engine which drives a vehicle. The vehicle has an engine control unit, which in each engine cycle, that is to say in each operating cycle of the spark ignition engine, determines a target ignition point for each spark plug. In order to generate an ion current, besides the ignition voltage source, disclosed as an ignition coil in US 2011/0016952 A1, a second voltage source is also provided, which continuously delivers a DC voltage between 30 V and 500 V. As a second voltage source, US 2011/0016952 A1 discloses a DC voltage transformer (DC-DC converter), which generates the desired second voltage from the on-board power supply voltage of 12 V. The second voltage applied continuously to the spark plug is superimposed by the ignition voltage during the ignition process, only, and is to enable a signal evaluation during the entire operating cycle of the spark ignition engine.
A high-voltage diode, which allows the ion current to pass, but protects the DC voltage transformer from the high ignition voltage, is arranged in the electrical line between the DC voltage transformer and the spark plug. A low-voltage diode, which allows the high ignition voltage to pass, but blocks the ion current, is arranged between the secondary winding of the ignition coil and the central electrode of the spark plug.
The ion current produced has to be amplified and filtered in a number of stages before it can be examined for anomalies, for example for knocking, auto-ignition or preignition.
The apparatus known from US 2011/0016952 A1 is relatively complex and cannot be integrated into an ignition coil. If the apparatus cannot be integrated into an ignition coil or if a common assembly cannot be formed from the ignition coil and the apparatus for generating and measuring ion current, the influence of interferences on the ion current signals increases and requires further filtering.
An apparatus for generating and measuring an ion current, which flows between the electrodes of a spark plug, is known from U.S. Pat. No. 6,498,490 B2. This apparatus has a capacitor which serves as a voltage source for the generation of the ion current and which is charged via the secondary winding of an ignition coil. The ion current that is to be measured and examined for anomalies flows through the secondary winding of the ignition coil. This not only has the disadvantage that the ion current produced has to be filtered in a complex manner, but also has the further disadvantage that the inductance of the secondary winding damps the ion current signal. The damping pertains particularly to the frequency range in which knocking occurs. The damping hinders the detection of anomalies of the combustion processes in the engine and in particular hinders the detection of knocking.
U.S. Pat. No. 7,878,177 B2 discloses an internal combustion engine, in which ignition coils and fuel injectors are powered by the same high-voltage source which generates the ion currents flowing between electrodes of ignition plugs. An engine control unit controls a plurality of switches which connect the high-voltage source with the fuel injectors, ignition coils and spark plugs at individual times.